The most commonly used, or "standard," envelope is the #10 envelope, which has dimensions of 4 1/8 by 9 1/2 inches and weighs 0.0208 ounces.
A regular envelope has a closed face and doesn't have an address window. This is the most commonly used envelope in business and personal mailings. The weight of the contents and envelope can affect the amount of postage required. Mail in a standard-size envelope that weighs under 13 ounces can be placed into a USPS drop box with the proper amount of postage; however, mail over that weight must be sent from an approved shipper location.